Refactor parent issues resolver

What does this MR do and why?

In !102348 (merged) I'll introduce a new issues query at the root level of the GraphQL API (more details on why at #377514 (closed)). In order to reuse resolver logic and all issue filters we first need to refactor the resolver's concern and split responsibilities so we can reuse most of it in the new resolver. Doing this in a separate MR to keep them smaller.

How to set up and validate locally

There should be no interface or spec changes as this is only a refactor. For verification you could make sure nothing has changed on the API definition and behavior around issue resolvers.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #377514 (closed)